This invention relates to mass polymerized rubber-modified monovinylidene aromatic copolymer compositions with an excellent balance of physical and mechanical properties and high intrinsic gloss, and methods for preparing such compositions.
Monovinylidene aromatic copolymers reinforced with rubber, in particular with diene rubber, represent a well known class of commercially available engineering polymers widely described in the literature. Specific examples of the copolymers are for example styrene and acrylonitrile copolymers, generally referred to as SAN resins, containing rubber particles, for example butadiene, dispersed in the polymeric matrix, generally known as ABS resins.
The rubber-modified monovinylidene aromatic copolymers can be prepared by continuous or batch processes and by various polymerization processes such as bulk, mass-solution, or mass-suspension, these are generally known as mass polymerization processes. A continuous mass polymerization process is known and described for example in U.S. Pat. Nos. 2,694,692; 3,243,481 and 3,658,946, and in published EP 400,479. This process consists of dissolving the rubbery material in the monovinylidene aromatic monomer and ethylenically unsaturated nitrile monomer mixture, adding possibly a radical polymerization initiator and an inert diluent, and then polymerizing the resulting solution. Immediately after the polymerization reaction commences, the rubbery material in the monomer mixture separates into two phases, of which the former, consisting of a solution of the rubber in the monomer mixture, initially forms the continuous phase, whereas the latter, consisting of a solution of the resultant copolymer in the monomer mixture, remains dispersed in form of droplets in said continuous phase. As polymerization and hence conversion proceed the quantity of the latter phase increases at the expense of the former. As soon as the volume of the latter phase equals that of the former, a phase change occurs, generally known as phase inversion.
When this phase inversion takes place, droplets of rubber solution form in the polymer solution. These rubber solution droplets incorporate by themselves small droplets of what has now become the continuous polymer phase. During the process, grafting of the polymer chains on the rubber takes place, too.
Generally, the polymerization is carried out in several stages. In the first polymerization stage, known as prepolymerization, the solution of the rubber in the monomer mixture is polymerized until phase inversion is reached. Polymerization is then continued up to the desired conversion.
Mass polymerization affords rubber-modified monovinylidene aromatic copolymers with a good balance of physical and mechanical properties, however the surface gloss of such copolymers is not always quite satisfactory. It is well know that surface gloss of a fabricated product is a function of copolymer composition as well as how the article is fabricated, e.g., the conditions under which the copolymer is molded or extruded. Less than favorable fabricating conditions result in lower surface gloss sometimes referred to as the intrinsic gloss of the copolymer.
The intrinsic gloss of diene rubber-modified monovinylidene copolymers can be improved by reducing the size of the rubber particles to less than 1 micrometer, e.g., by vigorous stirring during polymerization. However, this approach has not been successful because the usually available linear polybutadiene rubbers have a rather high molecular weight and thus a high solution viscosity, so that even with a strong agitation, it is not possible to achieve satisfactory rubber sizing, at least for the rubber concentrations commonly used in these copolymers (5 to 15 percent). Linear polybutadiene rubbers of low molecular weight and hence of reduced solution viscosity could be easily sized under stirring, but these rubber suffer from the known cold flow draw back, which introduced additional problems in their storage and handling.
U.S. Pat. No. 4,421,895 discloses the use of a diene rubber with a solution viscosity of 80 centipoise (cps) or less, when measured as a 5 weight percent solution in styrene at 25xc2x0 C., in ABS production. Specifically, the diene rubber proposed in this patent is a styrene and butadiene linear block copolymer. This type of block rubber does not suffer from cold flow and easily affords the formation of small particles. Using this type of linear block rubber and operating in accordance with the process described in this patent, an ABS with rubber particles of less than 0.7 micrometers (xcexcm) is obtained. However, by using the above styrene and butadiene linear block copolymer, the intrinsic gloss improvement is achieved at the expense of the other physical properties and mechanical properties, in particular the impact strength, so that the ABS obtained does not offer the desired combination of good physical and mechanical properties and intrinsic gloss.
Further, rubber-modified monovinylidene aromatic copolymers prepared from rubber mixtures of linear butadiene and linear block copolymer rubbers are disclosed for example in U.S. Pat. No. 5,756,579. However, while mechanical properties are improved these ABS compositions do not achieve the desired intrinsic gloss.
It is also known from the literature that rubber-modified monovinylidene aromatic copolymers can be prepared by using as the rubber a star-branched or radial block polymer. The use of such star-branched rubbers in rubber-modified monovinylidene aromatic copolymer production is discussed for example in U.S. Pat. Nos. 5,191,023; 4,587,294 and 4,639,494, in published EP 277,687 and in JP 59-232,140 and 59-179,611. However, while compared with other known diene rubbers, these star-branched rubbers afford copolymers with an improved balance of gloss and physical and mechanical properties they still have the drawback of lower intrinsic gloss.
Attempts to provide rubber-modified monovinylidene aromatic copolymers with an improved balance of gloss and physical and mechanical properties prepared from mixtures of star-branched and linear rubbers are disclosed for example in published JP 5-194,676; 5-247,149; 6-166,729 and 6-65,330 and in published EP 160,974. However, these compositions have large rubber particle sizes resulting in lower intrinsic gloss than desired.
In view of the deficiencies of the rubber-modified monovinylidene aromatic copolymer compositions thus obtained by utilizing any of such methods it would be highly desirable to provide an economical rubber-modified monovinylidene aromatic copolymer composition which exhibits an improved balance of physical and mechanical properties combined with high intrinsic gloss.
Accordingly, the present invention is such a desirable economical rubber-modified monovinylidene aromatic copolymer composition having a desirable balance of high intrinsic gloss and good impact resistance. The composition comprises a continuous matrix phase, comprising a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer, with a rubber dispersed therein as discrete rubber particles, said composition having an intrinsic gloss of at least 70 percent and an Izod impact strength of at least 150 Joule per meter (J/m). The rubber-modified copolymer is prepared using bulk, mass-solution or mass-suspension polymerization techniques.
In another aspect, the present invention is a process for preparing a mass polymerized rubber-modified monovinylidene aromatic copolymer composition comprising the steps of mass polymerizing in the presence of a dissolved rubber a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer, optionally in the presence of an inert solvent, to the desired degree of conversion and subjecting the resultant mixture to conditions sufficient to remove any unreacted monomers and to cross-link the rubber, said composition having an intrinsic gloss of at least 70 percent and an Izod impact strength of at least 150 J/m.
In a further aspect, the present invention involves a method of molding or extruding a mass polymerized rubber-modified monovinylidene aromatic copolymer composition comprising a continuous matrix phase, comprising a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer, with a rubber dispersed therein as discrete rubber particles, said composition having an intrinsic gloss of at least 70 percent and an Izod impact strength of at least 150 J/m.
In yet a further aspect, the invention involves molded or extruded articles of a mass polymerized rubber-modified monovinylidene aromatic copolymer composition comprising a continuous matrix phase, comprising a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer, with a rubber dispersed as discrete rubber particles, said composition having an intrinsic gloss of at least 70 percent and an Izod impact strength of at least 150 J/m.
The mass polymerized rubber-modified monovinylidene aromatic copolymer compositions of the present invention are especially useful in the preparation of molded objects notably parts prepared by injection molding techniques where a balance of high surface gloss and good impact resistance is desired. Such properties are particularly appropriate for household appliances, toys, automotive parts, extruded pipe, profiles and sheet for sanitary applications, power tool housings, telephone housings, computer housings, copier housings, etc.